Rationalized Fabrication of Structure-Tailored Multishelled Hollow Silica Spheres

Abstract

This work demonstrates a simple approach to rational fabrication of multishelled hollow silica spheres via periodic injections of a given amount of tetramethylammonium hydroxide (TMAH) to catalyze the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol/ water mixtures. Upon TMAH injection each time, the silica networks formed at the early stage of the sol−gel reaction of TEOS was found to possess noticeably lower condensation degree than these formed at the late reaction stage. The former could be readily and completely etched away in boiling water whereas the latter remained intact. The use of TMAH as basic catalyst led to temporally clear-cut separation between the early and late stages during the sol−gel reaction of TEOS and made the volume ratio of the loosely condensed silica gel networks, formed at the early reaction stage, to the densely condensed ones, formed at the late reaction stage well-defined around 3:2, enabling the formation of multishelled hollow spheres with precisely defined shell thickness and spacing between neighboring shells. The present template-free approach will open up new prospects in rational design and fabrication of structure-tailored multishelled silica spheres to accommodate to the demand of improved applications.

Single-step coating of mesoporous SiO2 onto nanoparticles: growth of yolk–shell structures from core–shell structures

“…52 The etching of mSiO 2 occurs when the -Si-O-Sinetwork is destroyed by OH − , following the three reaction equations in reverse direction. The possible reason that mSiO 2 can be etched from inside out is that the amount of Q2 (Si(SiO) 2 (OX) 2 , X = H or Et) is higher than the amount of Q4 (Si(SiO) 4 ) in the inner part of the mSiO 2 , 41 as has been found as well for the famous Stöeber process in which ultramicroporous silica particles can be grown 37,40,53,54 while the opposite is the case for the outer part of mSiO 2 . This makes the etching faster at the interior than at the exterior, and consequently mSiO 2 is etched from the inside, finally resulting in the formation of the yolk-shell structure.…”

Transformation of Stöber Silica Spheres to Hollow Nanocatalysts

“…Such as‐grown inhomogeneity has been utilized to make multi‐shelled hollow silica spheres (a type of HoMSs which was mentioned previously; Figure ii) . As reported in 2019 by Yang et al ., Stöber method also has the capability to produce multi‐shelled hollow structures via creating solution inhomogeneity in synthesis. During the Stöber process, one can gain control over the condensation of ethoxysilanol groups (such as Si(OEt) 3 OH, Si(OEt) 2 (OH) 2 , and even Si(OH) 4 ) and the re‐occurrence of condensation by interval addition of a weak base or structure directing agent (SDA) to control the growth of multiple silica shells.…”

“…As illustrated in Figure , various hollow structures can be obtained by transformation of Stöber SiO 2 spheres. Hollow silica spheres, multi‐shelled hollow silica sphere, mesoporous silica sphere with one‐dimensional (1D) channels ( m SiO 2 ), hollow m SiO 2 , hollow aluminosilica sphere, nanotubes‐assembled hollow copper‐silicate sphere, nanoplates‐assembled hollow nickel‐silicate sphere, hollow ZSM‐5 ( h ZSM‐5), and nanobubbles‐assembled manganese‐silicate hollow microbubble are just a few instances . Furthermore, desired hollow nanocatalysts can be prepared from their corresponding structures either by adding catalytic functionalities during the formation of hollow structures ( in‐situ functionalization) or by stepwise post‐treatments ( ex‐post functionalization) .…”

“… Schematic overview of hollow structures derived from Stöber SiO 2 spheres via various transformation methods. (i) hollow silica sphere, (ii) multi‐shelled hollow silica sphere, (iii) mesoporous silica sphere with 1D channels ( m SiO 2 ), (iv) hollow mesoporous silica sphere with 1D channels (hollow m SiO 2 ), (v) hollow aluminosilica sphere, (vi) nanotubes‐assembled hollow copper‐silicate sphere, (vii) nanoplates‐assembled hollow nickel‐silicate sphere, (viii) hollow ZSM‐5 ( h ZSM‐5), and (ix) nanobubbles‐assembled manganese‐silicate hollow microbubble …”

Small Structures Bring Big Things: Performance Control of Hollow Multishelled Structures

“…Among them, loading capacity and release rate are related to shell number, porosity, and pore size. [ 57,144–148 ] In general, for HoMSs of the same size, the one with more shells can achieve a larger drug loading capacity.…”